docs: def com_google_fonts_check_family_single_directory(fonts):
    """Checking all files are in the same directory.
  
    If the set of font files passed in the command line is not all in the
    same directory, then we warn the user since the tool will interpret
    the set of files as belonging to a single family (and it is unlikely
    that the user would store the files from a single family spreaded in
    several separate directories).
    """
  
    directories = []
    for target_file in fonts:
      directory = os.path.dirname(target_file)
      if directory not in directories:
        directories.append(directory)
  
    if len(directories) == 1:
      yield PASS, "All files are in the same directory."
    else:
      yield FAIL, ("Not all fonts passed in the command line"
                   " are in the same directory. This may lead to"
                   " bad results as the tool will interpret all"
                   " font files as belonging to a single"
                   " font family. The detected directories are:"
                   " {}".format(directories))